Table 6. Largest local sources of total nitrogen and total phosphorus and corresponding median incremental yields for National Hydrography Dataset catchments in the United States Pacific Northwest (2002 conditions).


[Catchments: Percentage of catchments where the source category was the largest contributor to the local load within catchments where local load is equal to the load generated exclusively within an National Hydrography Dataset (NHD) catchment. Median incremental yield: For catchments where the source was the largest contributor to the local load. Incremental yield is equal to the local load divided by the incremental catchment area. Abbreviations: kg/ ha-yr, kilogram per hectare per year; –, not applicable because source was not a source term used in this model or was the largest source in a small percentage of catchments; <, less than]


Largest local source category	Total nitrogen			Total phosphorus
	Catchments (percent)	Median incremental yield (kg/ha-yr)		Catchments (percent)	Median incremental yield (kg/ha-yr)
Atmospheric deposition	10.5	0.037		–	–
Urban sources1	2.92	2.47		1.40	0.42
Farm fertilizer	15.63	1.96		8.43	0.07
Livestock manure2	16.5	0.14		39.1	0.09
Geologic sources3	–	–		49.5	0.11
Forestland4	53.7	0.91		See footnote 6
Springs and power returns	< 1.0	–		< 1.0	–
Unidentified5	< 1.0	–		1.52	–

¹ Contribution from developed land, point sources, and nitrogen leaching from septic tanks.


² Contribution from all livestock manure.


³ Contribution from geologic phosphorus exported from forest, scrub, and grass land.


⁴ Contribution from asymbiotic nitrogen fixation in forests and symbiotic nitrogen fixation by red alder trees.


⁵ Catchments where no model predictions were made because they were internal sinks or, in the case of total phosphorus, there were no modeled nutrient sources.


⁶ Phosphorus export from forest land is included in the geologic phosphorus source category.

First posted July 17, 2013